iCal error while refreshing - can't find calendar

When adding a user's calendars from Zimbra into iCal we're experiencing the following error:

Error while refreshing

Calendar http://<server>/principals/users/<user>/ could not be found.

This happens right after setting up the account in iCal. It finds the calendars, because it populates iCal with the correct data, but then this error shows up and we can't do anything with the calendars after that.